Disclosed are a data transmission method and a terminal device. The method comprises: storing, by the terminal, a parameter value list; sending, by a terminal device, an first protocol data unit (PDU) session establishment request message to a session management function (SMF), wherein the first PDU session request includes at least one value in at least one of the parameter value list; and receiving, by the terminal device, a response message sent by the SMF, wherein the response message carries a rejection cause value to indicate the cause why the first PDU session is established unsuccessfully.